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After launching on 5 September 1977 and making fly-by encounters with Jupiter (1979) and Saturn (1980) Voyager 1 headed for the edge of heliosphere, which is the domain wherein the solar wind has dominance. Beyond the heliosphere lies the tantalising depths of the interstellar medium, made up of gas, charged particles and magnetic fields alien to our Solar System.

For the past decade Voyager 1 and its sister ship, Voyager 2, have been plunging deep through heliosphere towards its edge - a region known as the heliopause - where the pressure of the solar wind begins to lose strength as it is buffeted by the interstellar medium on the other side. Late last year scientists working on the Voyager mission, led by Stone, revealed that Voyager 1 had slipped onto a 'magnetic highway' where galactic cosmic rays - high energy particles from the interstellar medium - were entering the heliosphere while at the same time lower energy particles in the solar wind were leaking out into interstellar space. Little did they know that Voyager 1 had already reached interstellar space.

The trouble was that Voyager 1's plasma spectrometer, which could have measured the temperature, velocity and density of the interstellar plasma (that is, ionised gas), long ago broke down. Instead they were using the direction of the magnetic field as a proxy from which to infer whether Voyager 1 was still in the heliosphere. The magnetic field direction was not observed to have changed at all, suggesting Voyager 1 was still within the heliosphere. That all changed when a solar storm that burst out from the Sun in March 2013 finally caught up with Voyager 1 over a year later.

When in April 2013 the material from the Sun's coronal mass ejection passed Voyager 1, the spacecraft's plasma wave instrument detected the plasma around it shaking, vibrating like a guitar string. The frequency of the plasma's oscillations indicated a dense medium, forty times denser than Voyager 1 had experienced in the heliopause. At this distance only the interstellar medium could be this dense. This was it, realised Plasma Wave science team's principal investigator, Don Gurnett of the University of Iowa. Voyager 1 was in interstellar space.

In fact, when Gurnett's team looked back through archived data they discovered another set of faint oscillations in October-November 2012. By comparing the plasma densities of the two events, they calculated that Voyager 1 had actually entered interstellar space in August 2012, at a distance of 121 astronomical units from the Sun (one astronomical unit is the average distance between Earth and the Sun, which is 149.6 million kilometres)

The comparable locations of the two Voyager probes. Voyager 1, at top, has moved out of the heliopause, while Voyager 2 is still passing through it. Image: NASA.

"We literally jumped out of our seats when we saw these oscillations in our data - they showed us that the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," says Gurnett. "Clearly we had passed through the heliopause, which is the long-hypothesised boundary between the solar plasma and the interstellar plasma."

It should be pointed out that the solid bodies of the Solar System still extend beyond the heliosphere. The Oort Cloud, which is a sphere of icy bodies from which comets originate, may stretch up to a light year from the Sun, while the Sun's gravity clearly still dominates. So while it could be argued that Voyager 1 has not truly left the Solar System, the Sun's solar wind can no longer reach it and it is surrounded by the gas and particles that exist between the stars. It is in interstellar space.

To be precise, the Voyager team now say that Voyager 1 is in a transition region. It has cleared the heliosphere, but galactic cosmic rays are still not evenly distributed around the sky, indicating that the edge of the heliosphere continues to cast a shadow over proceedings. The next leg of Voyager 1's trek is therefore to move out into pristine interstellar space, unperturbed by our heliosphere, and the spacecraft will continue to send back data until at least 2020, and possibly up to 2025. Meanwhile, Voyager 2 is still within the heliosphere, 102 astronomical units from the Sun. It too will pass into interstellar space within the next few years.

The PlanetsFrom tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets. GET YOUR COPY

Hubble RebornHubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts. GET YOUR COPY

3D UniverseWitness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D! GET YOUR COPY